1. Field of the Invention
The present invention relates to a projection exposure apparatus used for forming fine patterns in semiconductor integrated circuits, liquid crystal displays, etc.
2. Description of the Related Art
A projection optical system used in a projection exposure apparatus of the type described above is incorporated in the apparatus after high-level optical designing, careful selection of a vitreous material, superfine processing of the vitreous material, and precise assembly adjustment. The present semiconductor manufacturing process mainly uses a stepper in which a reticle (mask) is irradiated with the i-line (wavelength: 365 nm) of a mercury-vapor lamp as illuminating light, and light passing through a circuit pattern on the reticle is focused on a photosensitive substrate (e.g., a wafer) through a projection optical system, thereby forming an image of the circuit pattern on the substrate. Recently, an excimer stepper that employs an excimer laser (KrF laser of wavelength 248 nm) as an illuminating light source has also been used for evaluation or research purposes.
Generally speaking, in order to faithfully transfer a fine reticle pattern to a photosensitive substrate by exposure using a projection optical system, the resolution and focus depth-of-field (DOF) of the projection optical system are important factors. Among projection optical systems which are presently put to practical use, those which are designed for the i-line include a projection optical system having a numerical aperture (NA) of about 0.6. In general, for a given wavelength of illuminating light, as the numerical aperture of the projection optical system is increased, the resolution improves correspondingly. However, the focal depth (DOF) decreases as the numerical aperture NA increases. The focal depth is approximately given by DOF=.+-..lambda./(2.times.NA.sup.2), where .lambda. is the wavelength of illuminating light.
Incidentally, the resolution is improved by increasing the image-side numerical aperture NAw (cf. the object-side numerical aperture NAr) of the projection optical system. Increasing the image-side numerical aperture NAw is the same as increasing the pupil diameter, i.e., increasing the effective diameter of an optical element, e.g., lens, which constitutes the projection optical system. However, the focal depth DOF decreases in inverse proportion to the square of the numerical aperture NAw. Accordingly, even if a projection optical system of high numerical aperture can be produced, the required focal depth cannot be obtained; this is a large problem in practical use.
Assuming that the wavelength of illuminating light is 365 nm of the i-line and the numerical aperture NAw is 0.6, the focal depth DOF decreases to about 1 .mu.m (.+-.0.5 .mu.m) in total range. Accordingly, a resolution failure occurs in a portion where the surface unevenness or the curvature is greater than DOF within one shot region (which is about 20 by 20 mm to 30 by 30 mm square) on the wafer.
In regard to these problems, super-high resolution techniques have been proposed, for example, a phase shift method such as that disclosed in Japanese Patent Application Post-Exam Publication No. Sho 62-50811, and a SHRINC (Super High Resolution by Illumination Control) method disclosed, for example, in WO92/03842, Japanese Patent Application Disclosure (KOKAI) No. Hei 04-180612 and Japanese Patent Application Disclosure (KOKAI) No. Hei 04-180613 (corresponding to U.S. Ser. No. 791,138 filed on Nov. 13, 1991). With these techniques, however, advantages such as an improvement in the resolution and an increase in the focal depth can be effectively obtained when a circuit pattern to be transferred is a periodic pattern having a relatively high density. However, substantially no effect can be obtained for discrete patterns (isolated patterns) such as those called "contact hole patterns" in the present state of the art.
In order to enlarge the apparent focal depth for isolated patterns, e.g., contact holes, an exposure method has been proposed in, for example, U.S. Pat. No. 4,869,999, in which exposure for one shot region on a wafer is carried out in a plurality of successive exposure steps, and the wafer is moved along the optical axis of the projection optical system by a predetermined amount during the interval between each pair of successive exposure steps. This exposure method is called FLEX (Focus Latitude Enhancement Exposure) method and provides satisfactory focal depth enlarging effect for isolated patterns, e.g., contact hole patterns. However, the FLEX method indispensably requires multiple exposure of contact hole images which are slightly defocusd. Therefore, a resist image obtained after development inevitably lowers in sharpness (steepness of the rise of the edge of the resist layer).
The Super-FLEX method published in Extended Abstracts (Spring Meeting, 1991) 29a-ZC-8, 9, The Japan Society of Applied Physics, is well-known as an attempt in increasing the focal depth during projection of a contact hole pattern without moving the wafer along the optical axis during the exposure operation, as in the case of the FLEX method. In the Super-FLEX method, a phase filter having a concentric amplitude transmittance distribution centered at the optical axis is provided on the pupil plane (i.e., a Fourier transform plane with respect to the reticle) of the projection optical system so as to increase the effective resolution and focal depth of the projection optical system by the action of the filter.
It should be noted that a method wherein the transmittance distribution or phase difference is changed by filtering at the pupil plane of the projection optical system to thereby improve the focal depth as in the case of the Super FLEX method, is generally known as "multifocus filter method". The multifocus filter is detailed in the paper entitled "Study of Imaging Performance of Optical System and Method of Improving the Same", pp.41-55, in Machine Testing Institute Report No. 40, issued on Jan. 23, 1961. The method of improving the image quality by spatial filtering at the pupil plane is generally called "pupil filter method".
The assignee has proposed as a new type of pupil filter a filter of the type that blocks light only in a circular region in the vicinity of the optical axis (this filter will hereinafter be referred to as "light-blocking pupil filter") in Japanese Patent Application Disclosure (KOKAI) No. Hei 04-179958 (corresponding to U.S. Ser. No. 76,429 filed on Jun. 13, 1993). The assignee has further proposed a pupil filter named "SFINCS" that reduces the spatial coherence of a bundle of image-forming rays from a contact hole pattern which passes through the pupil plane in U.S. patent application Ser. No. 128,685 filed on Sep. 30, 1993.
Separately from the above-described pupil filters for contact hole patterns, pupil filters which are effective for relatively dense periodic patterns, e.g., line and space (L&S) patterns, have also been reported, for example, in "Projection Exposure Method Using Oblique Incidence Illumination I. Principle" (Matsuo et al.: 12a-ZF-7) in Extended Abstracts (Autumn Meeting, 1991), The Japan Society of Applied Physics, and in "Optimization of Annular Zone Illumination and Pupil Filter" (Yamanaka et al.: 30p-NA-5) in Extended Abstracts (Spring Meeting, 1992), The Japan Society of Applied Physics. These filters are adapted to lower the transmittance (i.e., the transmitted light intensity) of a circular or annular region centered at the optical axis (this type of filter will hereinafter be referred to as "filter for L&S patterns"). In the L&S pattern filter method, the phase of light passing through the filter is not changed, unlike the Super FLEX method.
Among the foregoing various pupil filter methods, the Super FLEX method, the light-blocking pupil filter method and the SFINCS method enable the resolution and focal depth to be effectively increased with respect to isolated contact hole patterns among fine patterns which are to be transferred by exposure. However, for relatively dense patterns, e.g., L&S patterns, these methods cause the resolution to be undersirably low. Therefore, when such relatively dense patterns are to be exposed, it is desirable to unload the pupil filter from the projection optical system or to exchange it for a filter for L&S patterns.
However, the projection optical system is completed through a combination of high-level designing and production and strict adjustment to obtain a favorable projected image, as has been described above. Accordingly, if the pupil filter, which optically changes characteristics of the projection optical system, is merely loaded, unloaded or exchanged, the image-forming characteristics of the projection optical system are undesirably changed and cannot be favorably maintained.
In the case of an exposure apparatus designed on the premise that it will be used only for specific patterns, e.g., contact hole patterns, the projection optical system may be adjusted with a specific pupil filter incorporated thereinto when the system is set up, as a matter of course. However, in reality, in production lines for semiconductor devices or the like, a single exposure apparatus is used for pattern transfer by exposure at various steps in order to increase the production efficiency in the present state of art.